Creatures have CR level, not level. So its already not D&D. But I have to say - I DM a lot, and I've been in a few games of D&D; DMs do this stuff all the time for parties. Instead of scratching yams out of the dirt on your peasant farm and watching a Nautilus crash, you were on it, in Avernus, with Red Dragons and Gith attacking. Is that reasonable? No. Was it more fun? Yes. Why did the Gith ride off on the red dragon at the fallen bridge instead of just incinerating you? Because its an encounter and I can TPK you any second I want to, that isn't fun. I wanted it to be a Gith fight.
The only thing I would say is that creatures using correct CRs from MM and other source would be simpler for everyone and right now they might just be throwing the MM out the window and doing whatever they want. That same red dragon in 6 levels will have 2000hp, who knows? So, without rhyme or reason, we can only say that they are doing what they think is fun. The Arch Druid thing would make the grove storyline impossible, they'd be too powerful. If it was a D&D campaign and I was running it, yeah there wouldn't be an arch druid - thats the hook, its the noobs running it doing stupid stuff.
Conceptually, everything they did is fine. Divorcing it from the MM is annoying and the composition and titles of the NPCs could probably be better arranged so when you run into an actual max level arch druid it means something. You shouldn't have different levels of Arch Druid. A level 6 druid is running the place in place of Halsin, a Level 10. Everyone else is level 4. The watchers were level 5 shadow druids. There, now theres a lot of power swings depending on who you woo and its more dynamic.
Ultimately, they aren't approaching this like a DM and it shows.
